1. Field of the Invention
The present invention relates to novel organopoly-siloxane compositions of low viscosity, i.e. less than 55 000 mPa·s and preferably less than 30 000 mPa·s, intended to generate a foam of silicone elastomer (also known as “silicone foam”) with good physical and mechanical properties and of low density, i.e. less than 0.35 g/cm3 and preferably less than 0.25 g/cm3.
2. Description of Related Art
The expression “silicone foam” denotes a polyorganosiloxane composition in the foam form. Silicone foams are well known in the art and their preparation is described in a certain number of patents.
The expression “crystalline silica” denotes a silica in its natural form, of which quartz is one of the most widely known and used forms, in contrast to cristobalite and tridymite, which are much less widely used forms.
The expression “fumed silica” denotes silicas prepared by hydrolysis at high temperature (pyrohydrolysis), in a flame, of silicon tetrachloride SiCl4 according to the following reaction:

This chemical reaction releases a considerable amount of heat which is evacuated in a cooling zone. The only by-product is hydrochloric acid, which is separated at the outlet of the process and recycled so as to form, by reaction with silicon, silicon tetrachloride.
With regard to silicone foams, several techniques exist for producing them. A first technique employs a condensation reaction with release of volatile by-products. This is the case in particular for systems using the condensation reaction of the SiH—SiOH type, which makes it possible to release hydrogen which then acts as a porogenic agent. For example, French patent No. FR-A-2 589 872 describes a silicone foam precursor composition comprising an organosilicon polymer comprising siloxane units having hydroxyl groups bonded to the silicon, an organosilicon polymer comprising siloxane units having hydrogen atoms bonded to the silicon, a catalyst, for example a tin compound, and a finely divided filler comprising silica which has been treated to become hydrophobic. These compositions cure via a polycondensation reaction and, although they are satisfactory in many respects, the tin-catalyzed compositions described in French patent No. FR-A-2 589 872 are regarded as rather unsatisfactory owing to the use of a tin catalyst, which may exert certain undesirable toxic effects.
A variant described in U.S. Pat. No. 3,923,705 consisted in providing compositions comprising polydiorganosiloxanes bearing hydrogen atoms bonded to the silicon available for a condensation reaction with polydiorganosiloxanes bearing hydroxyl groups bonded to the silicon (silanols) in the presence of a platinum catalyst. This reaction thus makes it possible to construct the network while producing hydrogen gas necessary for the formation of a silicone foam. In this type of formulation, the formation of gas is proportional to the rate of crosslinking and consequently the density of the foams obtained is difficult to control, thus explaining the difficulties in obtaining low-density foams by this technique. These compositions can also comprise a polydiorganosiloxane bearing vinyl groups bonded to the silicon which crosslink simultaneously via polyaddition reactions with the polydiorganosiloxanes bearing hydrogen atoms bonded to the silicon, thus participating in the construction of the network of the silicone foam.
According to another variant described in U.S. Pat. No. 4,189,545, silicone foams are prepared from a composition comprising water, a polydiorgano-siloxane bearing vinyl groups bonded to the silicon, a polydiorganosiloxane containing hydrogen atoms bonded to the silicon and borne by units in the chain and not exclusively at the chain end, in order to be able to act as a crosslinking agent. The water reacts with the polysiloxane comprising hydride functions, thus producing hydrogen gas and a silanol. The silanol then reacts with the polydiorganosiloxane comprising hydride functions via a condensation reaction, thus generating a second molecule of hydrogen gas, while another polydiorganosiloxane bearing vinyl groups bonded to the silicon will simultaneously react, via an addition reaction, with another polydiorganosiloxane comprising a hydride function, thus participating in the construction of the network of the silicone foam. The main contribution made by this technique is that the hydrogen gas is produced without the addition of silanol and with the addition of a small amount of water.
In U.S. Pat. No. 4,590,222, silicone foams are prepared from a composition comprising a polydiorganosiloxane, a resin, a platinum-based catalyst, an organohydro-siloxane, a polyorganosiloxane bearing hydroxyl groups on the chain-end units, a filler and an organic alcohol.
However, techniques which use silanol as a source of porogenic agent have a tendency to give foams having densities which are too high for many applications, for example those intended for the transport industry. Furthermore, when medium-density foams are obtained, this most commonly occurs to the detriment of the mechanical properties (tensile strength, tear strength, etc.).
Another technique consists in using porogenic agents or additives, added to the silicone matrix, which, under the action of heat, expand the material:                either by decomposition with release of gas, the case in particular of derivatives of azo type, for example azodicarbonamide, which will make it possible to release nitrogen, carbon dioxide gas and ammonia. This type of porogenic agent, despite the fact that it is widely used for other materials, presents serious problems of toxicity (release of hydrazine),        or by a phase change (liquid to gas)—the case in particular of low-boiling-point solvents.        
Finally, an alternative technique consists in mechanically introducing a gas (for example, nitrogen) into the silicone matrix under pressure, followed by passage into a dynamic mixer, which makes it possible to obtain foams having good characteristics; however, they require bulky and expensive equipment.
Thus, despite the existence of the numerous techniques mentioned above, the production of low-density (less than 0.35 g/cm3 or 0.25 g/cm3) silicone foams from a composition of relatively low viscosity or of low viscosity (viscosity less than 55 000 mPa·s or than 30 000 mPa·s) still remains a problem which arouses the interest of silicone producers. For example, U.S. Pat. No. 4,418,157 describes silicone foam precursor compositions exhibiting, before crosslinking, a viscosity of less than 100 000 mPa·s. As is indicated in that patent, it is known (see column 2, lines 13 to 24) that the greater the viscosity of the composition, the less dense the resulting foam. Thus, an advantageous approach is described in that patent which consists in preparing a composition having a viscosity of less than 100 000 mPa·s and comprising a silicone base capable of crosslinking by polyaddition or polycondensation, to which are added a silicone resin of “MQ” type (nomenclature of the silicones as described, for example, in the work by Walter Noll “Chemistry and Technology of Silicones”, Academic Press, 1968, 2nd edition, on pages 1 to 9), optionally comprising vinyl functions, and water, which is described as a key constituent for the creation of hydrogen gas as described above. The addition of this specific resin makes it possible to lower the density of the foam obtained, even though the precursor composition is of low viscosity. However, this type of resin is an expensive raw material, the industrial synthesis of which requires bulky and expensive equipment.
Another example of a silicone foam precursor composition is described in the reference WO-A-00/46282. The composition described comprises a silicone base which crosslinks via a polyaddition reaction (polyorganosiloxane oil comprising an ≡SiH function/polyorganosiloxane oils comprising an ≡SiVi function/Pt catalyst, with Vi=vinyl group), a compound comprising a hydroxyl function and wollastonite (the examples describe compositions with high levels of fillers, approximately 21 parts by weight of fillers relative to the total weight of the composition). It should be noted that the viscosities of the compositions prepared in the examples (example 1, table 2) are all greater than 190 000 mPa·s. As is indicated above, it is known (see U.S. Pat. No. 4,418,157, column 2, lines 13 to 24) that the greater the viscosity of the composition, the less dense the resulting foam. It will be noted that, from the most viscous composition (reference WO-A-00/46282, table 2, page 13, composition [1-1], viscosity of 274 000 mPa·m) to the least viscous composition [1-3] (viscosity=198 000 mPa·s), the density of the foam obtained increases (from 0.20 g/cm3 to 0.25 g/cm3), thus confirming the known teaching relating to the difficulty in obtaining low-density foams from compositions of low viscosity (viscosity less than 55 000 mPa·s or than 30 000 mPa·s) before crosslinking. In point of fact, for reasons of optimization with regard to the use of these compositions, either by the end user or by manufacturers using silicone foam production lines, it is vital to be able to have a composition which, before crosslinking, is in a form of relatively low viscosity which readily flows in the appropriate tools.
Another problem encountered in the prior art foams relates to the sizes and the size distributions of the bubbles in the silicone foam material. Indeed, when said bubbles are too large, they lead to anisotropy of the physical properties according to the points of measurement. The expression “anisotropy of the physical properties” is intended to mean a variation in the values measured according to the point of measurement of the silicone foam.
The expression “bubbles of small size” for a silicone foam is intended to mean bubbles of which the width (or diameter) is less than or equal to approximately 1 mm, the expression “bubbles of medium size” is intended to mean that the width (or diameter) is between 1 and 1.5 mm, whereas, for “bubbles of large size”, the width (or diameter) is greater than 1.5 mm.
For example, document WO 2007/141250 describes an organopolysiloxane composition which is a precursor of a silicone foam, comprising:                at least one polyorganosiloxane (A) exhibiting, per molecule, at least two C2-C6 alkenyl groups bonded to the silicon and having a viscosity of between 10 and 300 000 mPa·s,        at least one polyorganosiloxane (B) exhibiting, per molecule, at least two hydrogen atoms bonded to the silicon and preferably at least three ≡SiH units and having a viscosity of between 1 and 5000 mPa·s,        a catalytically effective amount of at least one catalyst (C) composed of at least one metal belonging to the platinum group,        at least one porogenic agent (D) chosen from the group consisting of alkanols comprising from 1 to 18 carbon atoms,        optionally at least one inorganic and/or metal filler (F) which may be a fumed silica which is described as being generally characterized by a specific surface area of between 20 and 300 m2/g,        optionally at least one additive (G), and        with the additional condition that the choice, the nature and the amount of the constituents are determined such that the viscosity of said composition has to be less than 50 000 mPa·s, preferably less than 35 000 mPa·s and even more preferentially less than 15 000 mPa·s.        
However, the compositions exemplified all contain diatomaceous earths which do not make it possible to simultaneously obtain good storage stability and a homogeneous foam (problem of anisotropy of the physical properties according to the points of measurement).
For some applications, such as pad printing (or roller printing), specific properties of silicone foams are sought. Indeed, pad printing is an indirect printing process. The pattern to be printed is pre-engraved onto a backing, the plate is then attached to a pad-printing machine, and then the ink is deposited on the engraved parts in order to transfer the pattern onto the object to be printed by means of a pad or a roller made of silicone foam. In order to obtain an engraving and print of quality, it is essential that the pad or the roller made of silicone foam consists of bubbles of small sizes (the width or diameter is less than or equal to approximately 1 mm) and that the size distribution of the bubbles within the material be homogeneous so that the ink can be deposited and transferred uniformly onto the recipient layer backed by the layer of silicone foam while at the same time allowing a precise reproduction of the engraving. Thus, the need to obtain foams having bubbles of small size and a homogeneous bubble size distribution is particularly sought for this application.
Furthermore, the silicone foam industry is always seeking new compositions, which are silicone foam precursors, having a low viscosity, i.e. less than 55 000 mPa·s or than 30 000 mPa·s at 25° C., and capable of exhibiting good physical properties after crosslinking.
All the viscosities with which the present account is concerned correspond to a dynamic viscosity quantity which is measured, in a manner known per se, at 25° C. The viscosities are measured using a Brookfield viscometer according to the instructions of the AFNOR NFT 76-102 standard. These viscosities correspond to a “Newtonian” dynamic viscosity quantity at 25° C., i.e. the dynamic viscosity which is measured, in a manner known per se, at a shear rate gradient which is sufficiently low for the viscosity measure to be independent of the rate gradient.
However, when the formulation of low-viscosity compositions which have siliceous reinforcing fillers in order to improve the mechanical properties is attempted, one of the major problems encountered is the appearance of settling, which is observed especially after storage for a few months. Indeed, this phenomenon is observed when these compositions are stored, for example, in the form of a two-component system (or more commonly known as “RTV-2” system) for compositions which can foam at room temperature. Indeed for reasons of reactivity (crosslinking and foaming) and safety, the components are placed in two distinct parts in order to separate the catalysts and the porogenic agent comprising a hydroxyl function from the silicone oil comprising an SiH group. These settling phenomena make the composition unusable for certain applications.
The problem considered here can therefore be summarized as the search for a technical compromise between specifications, a priori contradictory, for the preparation of a composition having a low viscosity, i.e. less than 55 000 mPa·s or than 30 000 mPa·s, no longer exhibiting a settling problem when a siliceous reinforcing filler is used and which is a precursor of a silicone foam which has a low density, i.e. less than 0.35 g/cm3 and preferably less than 0.25 g/cm3, with good mechanical properties, bubble sizes of the order of less than or equal to 1 mm and a homogeneous bubble size distribution within the foamed material.